Such devices are available as a fixed frame with a movable base connected to a back, also movable, provided with an attachment in the form of a head-rest. It so happens that in the first position, where the base is moved completely back, the user finds himself in the normal sitting position, and the angle between the back and the base is close to 90.degree.. The head-rest is then generally retracted, since the user is in the active position. Should the user wish to assume a more relaxing position, he moves the base forward through a movement of his body, whereby the angle between the back and the base is increased above 90.degree., while at the same time, the head-rest moves to become an extention of the back. An ergonometric problem remains however, since the user is in a semi-inclined position and is supported only at the upper part of his back and, further down, by the base, while no support is provided at the small of the back.